It is previously known to voltage-test an electrically insulating film by passing it between two rolls which are under voltage with respect to each other and which are urged with a certain force towards each other (see German Offenlegungsschrift No. 2438094). However, when it is a question of an insulated metallic foil for a sheet winding, it is not sufficient only to test the insulating films, since an insulating film which is perfect per se may become damaged during the winding process by particles or burrs on the metallic foil cutting through the insulating films.
The present invention aims to provide a method of, and means for, controlling the manufacture of a sheet winding of the kind referred to, in which insulation faults, if any, may be detected and be repaired at the time of winding. The invention also aims to provide such a method, or means, which can be employed with relatively wide conductive metallic foils without the uncertainty factors associated with prior art methods.